The trend toward miniaturization of computing hardware, peripherals, as well as of sensors, detectors, and image and audio processors, among other technologies, has helped open up a field sometimes referred to as “wearable computing.” In the area of image and visual processing and production, in particular, it has become possible to consider wearable displays that place a very small image display element close enough to a wearer's (or user's) eye(s) such that the displayed image fills or nearly fills the field of view, and appears as a normal sized image, such as might be displayed on a traditional image display device. The relevant technology is commonly referred to as “near-eye displays.”
Near-eye displays are fundamental components of wearable displays, also sometimes called “heads-up displays.” Emerging and anticipated uses of wearable displays include applications in which users interact in real time with an augmented or virtual reality. Such applications can be mission-critical or safety-critical, such as in a public safety or aviation setting. The applications can also be recreational, such as interactive gaming. The applications of wearable displays are expected to grow as the technology improves.
Among the challenges of creating heads-up or wearable displays are size and weight of the display components and of the integrated display made from them. For practical and/or anticipated applications, it is desirable for the components of wearable displays to be small and light-weight. To help pave the way for consumer adoption of wearable displays, it is further desirable that the components be relatively inexpensive and easy to manufacture.
One of the physical components of a heads-up display is an optical transport element that delivers an image, such as one generated by a graphics processor or the like, to one or both of the wearer's eyes. In addition to meeting the challenges of size, weight, and manufacture, the optical transport component should have optical characteristics that efficiently couple light in and out of the component and retain the fidelity of transported images.
Existing components having adequate optical performance characteristics have either been too large or too expensive and difficult to manufacture for practical use, or both. Conversely, components made sufficiently small or cheaply to be considered practical have forfeited too much in the way of optical performance. The devices so far devised for possible use as optical transport in a wearable display fall short of coupling-efficiency and/or image-quality requirements, are too large or heavy for integration into a wearable display, or are too expensive or too difficult to manufacture on a practical scale.
One approach has been to use dichroic mirrors to reflect different wavelength ranges of light into and out of a waveguide. However, the size and weight of such mirrors has resulted in a bulky optical component. Moreover, dichroic mirrors require very precise deposition of dielectric multilayer films. This, in turn, translates into high cost and difficulty in mass production, which, combined with the sub-optimal size and weight characteristics, makes dichroic mirrors an unattractive option for coupling elements in wearable displays.
Another approach has been to use a diffraction grating for the in-coupling and out-coupling elements. While this may make it possible to accommodate the size and weight requirements of a waveguide for wearable displays, diffraction can result in color separation and other forms of image degradation. Additionally, since the size scale of diffractive elements of a grating are similar to the optical wavelengths being diffracted, high precision—and correspondingly high cost—is required in manufacture. Alternatively, lower manufacturing cost may be achieved at the expense of precision, but also at the expense of coupling efficiencies and optical performance (i.e., image quality).